FIG. 1a shows with a simplified block diagram a GSM network (Global System for Mobile communications).
The network subsystem NSS 100 of the GSM system comprises a mobile services switching center MSC 101 connected to other networks through the interface of the NSS, such as a public-switched telephone network PSTN, an integrated services digital network ISDN, public land mobile networks PLMN, packet-switched public data networks PSPDN, and circuit-switched public data networks CSPDN. A base station subsystem BSS 102 is located between A and air interfaces comprising base station controllers BSC 103, each controlling the base transceiver stations BTS 104 connected to them. The base stations, on the other hand, are in radio communication with mobile stations MS 105 across the air interface.
The GSM network has a digital radio path, i.e. bits are transferred through the air interface. The GSM system uses coding to convert speech into bits and decoding to reconstruct speech back into its original form. A transcoder TC (not shown in the figure) located in the BSS encodes the downlink transmissions, and a mobile station encodes the uplink transmissions. By using the coding system, speech coding requires only 13 kbit/s instead of the 64 kbit/s used in other networks, e.g. PSTN.
In order to establish a call, a mobile station MS must have a connection through the air and the base station system. This connection may be either a signaling type of connection or a traffic (speech, data) type of connection. Call control functions are handled by the NSS part of the GSM network. Via the base station system, the mobile switching center provides the connection between the mobile station and the external networks.
FIG. 1b shows some of the messages, which are generally used at call establishment between a calling and a called party.
In the following the call establishment of a mobile originated call is considered. When a calling party MS wishes to request a service, the user first dials the number and lifts the handset of the phone, or pushes a button when a mobile phone is in question. A REQUEST-FOR-SERVICE signal 100 is sent on a random access channel RACH to the network, indicating that the calling party requires a speech channel. For the sake of simplicity, signaling is performed between the calling party and a mobile switching center MSC. In fact, a number of signaling message exchanges are required among the mobile station, the serving base station BTS, and the serving base controller station BSC before signaling reaches the serving mobile switching center. Parameters such as the power level and the channel number are determined. Mobile switching center/base controller station responses with a free channel number and a REQUEST-ADDRESS message 101 are sent from the mobile switching center to the mobile station. The mobile station is designated a dedicated channel. The message 101 is an inquiry about the number of the called party. The mobile station responds by sending a PROVIDE-ADDRESS message 102, including the number in question, to the mobile switching center. Information is processed in the mobile switching center 103. The processing includes routing to the destination, initialization of billing, HLR (Home Location Register) and VLR (Visitor Location Register) verifications, allocation of a speech channel, etc.
It is assumed that the destination of the call, the called party, is in this particular example an extension number. In this case, an ALERT-CALLED-PARTY message 104 is sent from the mobile switching center to the switchboard of the called party. The switchboard checks whether the called party, the extension number, is ready to receive the incoming call. This happens so that an ALERT-CALLED-PARTY message 104, containing a call line indication CLI, is sent further to the extension number of the called party. This message activates a ringing tone in the equipment of the called party. If the called party is already engaged in communication, at step 105, notification is given with a busy tone 106. First, the switchboard informs the calling subscriber of this by sending a SWITCHBOARD-ANSWER message 107 through the signaling channel, and a traffic channel is activated for speech at the mobile switching center. Then a switchboard operator and the calling party discuss the matter. If the calling party decides to wait for the service, the switchboard operator switches on a queue tone or a piece of music.
Generally speaking, the waiting time varies for different services, but from the subscriber's point of view, the waiting usually seems to be too long. Service providers have solved this problem in various ways. Information can be given regularly to the calling party with a recorded voice saying that the company has not yet been able to complete the call. More often a queue tone or alternatively a piece of music is played on the line to indicate that the calling party is on queue for the service.
From time to time the switchboard tries to contact the called party by recalling, steps 109-112. Finally, when the called party answers by lifting the handset of her/his telephone, an OK message 113 is sent from the telephone equipment to the switchboard. The call connection is established between the calling and the called party, and the conversation can begin 114. The conversation can be terminated at any moment when either of the two subscribers hangs up 115.
Depending on the service provider, a busy tone, a queue tone, or a piece of music is sent to the calling party when the destination is found to be already busy. It is especially important to note that the queue tone or piece of music is sent through the air to the mobile station in the same manner as speech. That is, the piece of music is sent on a TCH channel (Traffic CHannel). This is, however, wasting a lot of spectrum. It is a known problem that the frequency range is limited. Therefore, it is not economical to use the frequencies only for sending either a queue tone or a piece of music on a speech channel. The limited scope of the frequencies and the continually increasing need to share out frequencies among network subscribers requires that the frequencies be used in a more efficient and economic way.
Another problem is the poor quality of music when it is sent through a radio path. One important reason for the poor sound quality is that the piece of music is compressed and coded into bits in the same manner as normal speech; originally compressing and coding was optimized for speech.
In particular, all kinds of conversions degenerate the tone quality. When the subscriber listens to a piece of music of poor quality, the impression may be the opposite to that which the service provider desires. Still another problem is that the subscriber may not necessarily want to listen to that certain piece of music but can't in any way act to change the situation.